Melt the chocolate in a bowl set over a saucepan of simmering water.
Combine the remaining ingredients, except for the hazelnuts, in a food processor or mixer and blend for 1 minute at low speed.
Add the melted chocolate and blend for 1 minute at high speed.
Fold in the hazelnuts.
Lightly grease a 1.2 litre pudding basin with margarine.
Spoon the batter into the basin.
Cover with a lid or a pleated greased sheet of foil tied securely with string.
Place the basin on a rack set in the bottom of a saucepan.
Pour boiling water into the saucepan until it comes three-quarters of the way up the side of the basin.
Cook over a medium heat for about 1 1/2 hours or until a knife or skewer inserted into the centre of the pudding comes out clean.
Remove the pudding from the saucepan and allow to cool for 10 minutes.
Run a knife around the edge to loosen and invert on to a serving dish.
Serve with Vanilla Custard.